1. Field of the Invention
The present invention relates to a method for performing a functional test of a position measuring system, which in the course of scanning a scale by a position measuring unit generates position values, and to a position measuring system for executing the method.
2. Discussion of Related Art
A position measuring system includes a position detection unit for the generation of positional data words, which are fed to a processing unit of the position measuring system and from which position values are generated in the processing unit. Here, the generation of the positional data words by means of a position detection unit takes place by scanning, for example optical scanning, of a scale (measuring graduation) by a measuring transducer device of the position detection unit during a relative movement of the position detection unit with respect to the scale. If, for example, the measuring transducer device is operated in a known manner in accordance with a magnetic or optical scanning principle, it includes magnetic detectors or optical detectors (photo-detectors), which in the course of scanning the associated scale (measuring graduation) generate electrical signals, which are passed on to the processing unit in the form of digital positional data words for generating position values.
By arranging the position detection unit, on the one hand, and the scale on the other, respectively on two objects which are movable with respect to each other, for example in the form of machine elements of a machine tool, it is possible to detect the respective position of the one object in relation to the other object, and this, as a function of the type of the scale, either in the form of an absolute position (when using a scale formed by a code track), or in the form of relative position changes (when using an incremental periodic measuring graduation of the scale). The position measuring system can be designed for longitudinal measurements, on the one hand or, on the other for angle measurements, depending on the type of the movement of the two objects in relation to each other.
To assure that the position values generated from the positional data words in the processing unit, which are to be passed on for evaluation to an evaluation unit (electronic follow-up device) arranged downstream of the position measuring system, are not distorted because of electronic malfunctions or interferences, a functional test is performed in the course of position measuring in that at least one control value is generated in the position measuring system in addition to the position values, which has a specific mathematical interrelationship with one of the position values, wherein a correction factor is introduced into this mathematical interrelationship, which is stored in a memory of the position measuring system.
The control value, together with the position values, is transmitted to the evaluation unit (electronic follow-up device) arranged downstream of the position measuring system, which can be, for example, a component of a machine control device, which controls a machine tool as a function of the position values generated in the position measuring system. For this purpose it is possible in connection with the evaluation unit and, in some applications even with the position measuring system itself, to form values composed of or derived from several individual position values such as, for example, speed values as a change of the individual position values over time, and acceleration values as a change of the individual speed values over time. In this case the term “position values” is also intended to include the values respectively composed of or derived from several individual position values, such as speed, acceleration and jolts. However, the main field of application is the transmission of individual position values in the narrower sense from the position measuring system to the evaluation unit, so that then possibly further values, such as speed and acceleration, for example, can be formed from them, which are required for the control or regulation of a machine.
A check is also made in the evaluation unit (electronic follow-up device) whether the expected mathematical interrelationship exists between the control value and the associated position value, and this is done by using the correction value stored in the memory of the position measuring system, which is transmitted to the electronic follow-up device for this purpose. Thus, the position values generated in a position measuring system, for example, are regularly charged with an offset in the form of a so-called zero point shift before the respective position value is passed on to the electronic follow-up device. By this it is possible to create a definite reference position (a so-called zero position) for every type of the arrangement of the position measuring system on the machine tool or the like. As a rule, a suitable zero point shift is defined in the course of the initial start-up of a position measuring system.
Besides the position values charged with the zero point shift, second position values, called control values, which are not charged with the zero point shift and each of which relates to the same measured position value (i.e. the same positional data word) as each (actual) position value charged with the zero point shift, can be created in the position measuring system. In this case the respective control value and the associated position value are preferably generated independently of each other.
By transmitting the position values charged with the zero point shift, as well as the control values not charged with the zero point shift, to an electronic follow-up device arranged downstream of the position measuring system, it is possible to perform there a comparison between the individual position values and a respectively associated (i.e. not related to the same positional data word generated by the measuring transducer device) control value. By comparing control values and position values which are associated with each other in the electronic follow-up device it is possible to check whether the expected mathematical interrelationship exists between them, and this while using the correction value introduced into this mathematical interrelationship, which is read out of the memory of the position measuring system for this purpose.
However, in the course of this functional test it is not possible to detect those errors which are based on a hardware defect of certain memory cells of the memory of the position measuring system. A distortion of the correction value because of a hardware defect in the memory of the position measuring system has the same effect on the generation of the control value in the position measuring system as during the subsequent check of the mathematical interrelationship between a position value and the associated control value in the downstream arranged electronic follow-up device.